Alternative splicing of FGFR2 exons IIIb and IIIc results in the cell type-specific expression of the FGFR2-IIIb isoform in epithelial cells and FGFR2-IIIc in mesenchymal cells and this splicing choice is essential during development. We previously hypothesized an epithelial cell-type specific splicing program in which FGFR2 and other transcripts is coordinated by epithelial-specific splicing regulators. In the first successful use of a genome-wide, high throughput cDNA expression screen for splicing factors, we discovered essential epithelial-specific FGFR2 splicing regulators and named them Epithelial Splicing Regulatory Proteins 1 and 2 (ESRP1 and ESRP2). Ectopic expression of either protein in mesenchymal cells switches FGFR2 splicing to the epithelial pattern, whereas depletion of both factors in epithelial cells via RNA interference has the opposite effect. Further identification of CD44, ENAH, and p120-ctn as splicing targets of ESRP1 and ESRP2 implicates these proteins as the regulators of a broader epithelial post-transcriptional gene expression program. We will further characterize the mechanisms by which the ESRPs regulate splicing of FGFR2 and identify additional co-regulated alternatively spliced transcripts that constitute an epithelial-specific splicing signature. Expression of both ESRP1 and ESRP2 is turned off during the Epithelial Mesenchymal Transition (EMT) and therefore we will also investigate the role of ESRP downregulation and the corresponding changes in splicing of co-regulated transcripts that contribute to the EMT. Specific Aim Number 1. Complete the identification of FGFR2 splicing regulators using established high throughput, cell-based splicing assays. We will complete genome-wide, high-throughput screens to establish a comprehensive set of splicing regulators that cooperate with the ESRPs in the general epithelial splicing program. Specific Aim Number 2. Investigate the mechanisms by which ESRP1 and ESRP2 regulate epithelial cell type-specific splicing. The molecular mechanisms by which the ESRPs regulate splicing will be investigated through the identification of ESRP binding sites, essential protein domains, and functionally relevant protein-protein interactions. Specific Aim Number 3. Identify an epithelial cell type-specific splicing signature of co- regulated alternative splicing events that are regulated by the ESRPs. Using splicing sensitive microarrays we will define a comprehensive set of biologically coherent transcripts that are co-regulated by the ESRPs in addition to FGFR2, CD44, ENAH, and p120-Ctn. Specific Aim Number 4. Investigate the role of the ESRPs in the Epithelial Mesenchymal Transition (EMT). We will determine whether the loss of ESRPs expression is required for the EMT to occur and whether ESRP expression sufficient to induce a Mesenchymal to Epithelial Transition (MET).